Porous refractory ceramics have long been used as particulate filters in hot gas or corrosive environments such as advanced coal-based gas turbine cycles, municipal and industrial waste incinerators, and diesel or natural-gas engine exhaust systems. For such applications, ceramic particulate filters must possess chemical inertness, thermal shock resistance, high filtration efficiency, low pressure drop, and adequate strength. In particular, a diesel particulate filter (DPF) ideally combines low CTE (for thermal shock resistance), low pressure drop (for engine efficiency), high filtration efficiency (for removal of most particles from the exhaust stream), high strength (to survive handling, canning, and vibration in use), and low cost.
Candidate materials for DPFs include cordierite, silicon carbide and aluminum titanate based ceramics. Cordierite is attractive due to its low cost, low CTE, and good strength. However, the relatively low volumetric heat capacity (approximately 2.8 J cm−3 ° C.−1 at 800K) and low thermal conductivity of cordierite can result in unacceptably high temperatures during operation when the filters are regenerated under certain conditions. Further, obtaining a well-interconnected pore microstructure in cordierite filters, in combination with low porosity required for high thermal mass, has been a challenge.
Silicon carbide filters have an advantage of a well-interconnected porosity for low pressure drop. Higher volumetric heat capacity (approximately 3.6 J cm−3° C.−1 at 800K) and high thermal conductivity, coupled with a very high melting point, make silicon carbide thermally durable. However, silicon carbide is relatively expensive. Furthermore, the high coefficient of thermal expansion requires silicon carbide filters to be fabricated as cement-bonded segments, adding to manufacturing cost and raising concerns about their long-term thermo-mechanical durability.
Aluminum titanate (AT) based ceramics and specifically mullite-aluminum titanate (MAT) ceramics offer a very high volumetric heat capacity (approximately 3.9 to 4.0 J cm−3° C.−1 for fully dense MAT at 800K) in combination with a low CTE. Further, AT and MAT ceramics have excellent chemical durability and high melting point.
However, in the manufacture of AT and MAT bodies, high sintering temperatures greater than 1600° C. are often required to achieve sufficient grain growth for microcracking and low thermal expansion. Such high heating temperatures add cost to manufacturing and final product. To reduce the sintering temperature some approaches have utilized the addition of chemical components. Nonetheless, such methods often result in a strong sensitivity of the physical properties, including CTE, porosity, or pore size, to the firing temperature, which is undesirable for manufacturability. Also, desired properties for DPF use are not expected to be achieved.
A need therefore exists to have an AT body that can be manufactured at lower sintering temperatures with properties that are useful for high temperature filtration applications. Further, there is a need for an AT body which has a narrow pore size distribution without sacrificing CTE, porosity, and strength.